The invention relates to a circuit layout for signal amplification of the type stated in the preamble of claim 1.
Linear operation amplifiers with high amplification must be capacitively coupled if the signal D.C. voltage is not defined or is so great that the amplifier limits on one side. With capacitive coupling, the centre voltage, which is superposed by the A.C. voltage, is applied to the amplifier input. The transmission of the A.C. voltage is influenced by the time constant .tau.=R.sub.e .multidot.C (where C=coupling capacity, R.sub.e= resistance value of the input resistance of the amplifier). To minimize signal distortions, the time constant must exceed the largest signal period. On the other hand, in order for the amplifier to return to its set operating point in finite time in the event of a high-amplitude interference signal, the time constant cannot be of arbitrary size.
Particularly in cases where the signal voltage is proportional to a light intensity, the result is very high dynamics and interference signals which may exceed the useful signal by a factor of 1000, for example. Single interference signals of this type lead to a high displacement current flowing in the coupling capacitor for a short period as a consequence of the finite time constant, with the result that the amplifier is overmodulated with opposite polarity following the interfering pulse until the capacitor has been reloaded. Interference of this nature may occur periodically during image transmission. In this case, the amplifier is driven to its limit on both sides, depending on the amplitude and the duty cycle of the interference signals. Information contained in the low-level signal is completely lost thereyb, as no suitable rest operating point is set at the amplifier input.
To reduce the influence of large interference signals, D.C. amplifiers, for example logarithmic amplifiers, with diodes in the negative feedback branch are frequently used. This reduces the amplification for high-level signals, but does not eliminate the shift in the operating point caused by high-level signal pulses in the case of capacitive signal coupling. This greatly reduces the amplification for low-level signals following an interference pulse until the operating point has reset.